This invention relates to a valve for controlling the passage of air, gas, a fluent medium (such as air carrying solid particulate material, liquid or a flowable particulate solid) along a duct or through an opening. Such valves are hereinafter referred to as being for the purpose specified.
The invention relates more particularly to isolating valves, sometimes referred to as dampers, for use primarily although not exclusively in pipes through which pulverised fuel is blown. The mixture of pulverised fuel and air presents a potential explosion hazard and thus isolating valves used in such pipe lines must be robust and provide a good seal. However, similar valves may be employed in pipe-lines carrying fluids in general (both gaseous and liquid) and solids in a flowable condition, whether entrained in a fluid or not.
Valves currently used for such purposes are of two main types, generally referred to as plate-type and goggle-type.
Plate-type valves consist of a plate or blade which is movable into and out of an aperture in a frame set across the pipe, the blade mating with the frame to form a seal. The blade movement is generally linear, and the seal efficiency generally relies on the machining tolerances of the component parts.
Goggle-type valves also have a blade sliding in a frame, but of a size such that throughout its range of movement it always registers with the pipe, the plate having an eccentrically situated hole approximately commensurate with the cross-section of the pipe. In the "open" position, the hole is in register with the pipe and in the "closed" position the hole is moved laterally away from the pipe so that the remainder of the blade isolates the pipe sections on either side. In both positions, the clearances between the goggle plate and the fixed frame have to be sealed. Sealing is in some cases effected by means of spring loaded sealing rings which maintain contact with the blade throughout its movement, but this causes wear to occur, and increased tractive effort is required to move the blade. In other cases, sealing is effected by rigid seatings afforded by manually or automatically moved duct flanges which are slackened to permit movement of the goggle plate and tightened again when the plate is in either of the two terminal positions. If this is done manually, much plant time can be wasted, whilst the provision of a second mechanical actuator to perform the tightening function adds significantly to the cost of the valve and also to the cost of any sequencing circuitry for automatic operation.
A third type of valve sometimes used operates on a rather different principle and requires a seating within the cross-section of the pipe and a closure member which is movable both in a lateral direction to bring it into or out of alignment with the seating, and also in an axial direction to bring it into or out of sealing engagement with the seating. This design has the disadvantage that the seating can obstruct smooth flow within the pipe and the operating mechanism is complicated because of the two-stage movement required, particularly since the axial movement has to be effected by operating means disposed within the duct and thereby subject to the material carried thereby.